Hi there,
what do I think as a developer? I think E-commerce or Shopify both you can use for this dropshipping purpose, You have mentioned that your budget is 300 - 600 but frankly speaking what do you need is 900-1000 to get actually what do you need, I am the owner of an IT consultancy and a full stack developer too, I can create this platform for you where you will be able to sell themes. please come over chat to discuss this in details. thank you
Ammar